Intrusion detection is becoming more important, both from a military standpoint and from a commercial activity. Military bases and installations, factories, power stations, stores and even private homes are being protected by sophisticated electronic surveilance systems which are capable of monitoring the periphery of the facility so as to warn against unwanted intrusion.
One type of intrusion detection which has become more important in this security conscious world is that where a sensing element is concealed in the ground around the outer edges of an area to be protected. The sensing element, of whatever type, is adapted to monitor the movement of persons or objects approaching the outer edge, usually producing a signal indicating such an intrusion.
To enhance the value of any intrusion detection system, it is desirable that the sensor be able to detect more than one form of intrusion. Specifically, magnetic intrusion is normally detected, such as for example, by detection of a mass of metal which breaks through or encroaches upon a magnetic field. Objects which might be detected by these devices are obvious metal products such as vehicles, weapons, tools and the like. Examples of patents which disclose sensors capable of detecting intrusion by metallic objects are U.S. Pat. No. 3,747,036 and U.S. Pat. No. 3,754,223.
While detection of intrusion of the aforementioned type is likely to give a reasonable degree of assurance, additional protection is desirable if it can be achieved without greatly increasing costs and technical difficulties. Obviously, watchmen or sentries can be placed around an area to be guarded, but this is an expensive and not always reliable safeguard. Remote control cameras and other visual sensors bring about additional costs. It would be desirable if an intrusion detection sensor could be developed which would permit the use of already existing magnetic detection apparatus. It would be particularly desirable if intrusion could be determined by recognition of pressure and/or seismic disturbances so as to intercept and detect nonmetallic objects moving across the line to be guarded.
Accordingly, it is an object of this invention to provide a transducer capable of recognizing both seismic and pressure signals.
Another object of this invention is to provide a transducer which is compatible with magnetic intrusion detection systems and which produces a signal responsive to both seismic and magnetic intrusion.
Yet another object of this invention is to provide a transducer for intrusion detection which is capable of being adjusted to vary the degree of sensitivity, so that the particular sensitivity can be selected for the particular use in mind.
Other objects will appear hereinafter.